Electrodeless low-pressure discharge lamp having a straight exhaust tube fixed on a conical stem

ABSTRACT

Electrodeless low-pressure discharge lamp having a lamp vessel sealed in a gas-tight manner and filled with a metal vapor and a rare gas. The lamp vessel comprises a glass envelope which is connected in a gas-tight manner to the edge of a conically widening collar at the end of a tubular part of a sealing member also made of glass. The tubular part accommodates a rod-shaped core of a magnetic material by means of which a discharge is generated in the lamp vessel during operation of the lamp. An exhaust tube extends parallel to the longitudinal axis of the tubular part terminating at the wall of the conical collar. The end of the exhaust tube engages round an orifice in a planar surface in a cam-shaped recess provided in the wall of the conical collar. The planar surface extends substantially at right angles to the longitudinal axis of the tubular part of the sealing member.

BACKGROUND OF THE INVENTION

The invention relates to an electrodeless low-pressure discharge lamphaving a lamp vessel which is sealed in a gas-tight manner and is filledwith a metal vapor and a rare gas. The lamp vessel comprises a glassenvelope which is connected in a gas-tight manner to the edge of aconically widening collar at the end of a tubular part of a sealingmember also made of glass. The tubular part accommodates a rod-shapedcore of a magnetic material by means of which a discharge is generatedin the lamp vessel during operation of the lamp. The known lamp has anexhaust tube which extends parallel to the longitudinal axis of thetubular part of the sealing member and terminates at the wall of theconical collar. A lamp of this type is known from "Neues aus derTechnik", 1986, 1.

The sealing member in the known lamp is in the form of a stem which isconnected in a gas-tight manner by means of sealing glass to theenvelope-shaped part of the lamp vessel. The use of a stem in itself isquite conventional in tubular low-pressure mercury discharge lampsprovided with electrodes and in incandescent lamps. In these lamps thestems are provided with electrodes and a tubular exhaust tube whichterminates in a so-called pinch which is located on or near thelongitudinal axis of the stem. An exhaust tube is required forexhausting the lamp vessel, that is to say, filling the lamp vessel witha desired rare gas atmosphere, degassing of electrodes, the wall andother components of the lamp vessel, etc.

However, due to the presence of the rod-shaped magnetic core there is nospace for an exhaust tube in the centrally located tubular part of thesealing member formed as a stem in an electrodeless lamp. However, theselamps also require exhausting of the lamp vessel.

In the known lamp the exhaust tube is therefore connected to the conicalcollar of the sealing member. For the purpose of satisfactory progressof exhausting in a bulk manufacturing process it is desirable that anexhaust tube extends in the direction of the longitudinal axis of thelamp vessel (the same direction as the longitudinal axis of the tubularpart). When the end of an exhaust tube engages a wall which extendsobliquely with respect to the said longitudinal axis, it is necessarythat prior to or subsequent to fixation to the said wall part theexhaust tube is bent until the correct position (in the direction of thelongitudinal axis) for exhausting is reached. This bending process is anadditional time-consuming step in the manufacturing process creating aready risk of breakage of the sealing member or the exhaust tube.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrodelesslow-pressure in a bulk manufacturing process in which theabove-mentioned drawbacks of the known lamps are obviated.

According to the invention, an electrodelss low-pressure discharge lampof the type described in the opening paragraph is thereforecharacterized in that the wall of the conical collar is provided with acam-shaped recess having a bottom part which is a planar surfaceperpendicular to the longitudinal axis of the tubular part of thesealing member. The end of an exhaust tube is connected to theperpendicular surface and the exhaust tube is aligned with thelongitudinal axis of the tubular part. An orifice in the perpendicularsurface allows communication between the exhaust tube and the innerspace of the lamp vessel.

The advantage of the lamp according to the invention is that the exhausttube can be fixed in the correct position to the conical collar withoutextra operations during the manufacturing process. It has been foundthat the risk of breakage in the glass wall of the collar around thepoint of fixation of the exhaust tube is small during manufacture of thelamps.

In the manufacture of the lamp the cam-shaped recess is firstly providedin the conical wall part by locally heating the glass over a smallsurface area. The recess having the perpendicular planar surface isformed by means of an abutment. Subsequently the end of the exhaust tubeis connected to the bottom part by softening the end of the exhaust tubeand an opening is provided in the bottom part. The exhaust tube is thenpositioned in the vertical direction, that is to say, parallel to thelongitudinal axis of the lamp. Subsequently the exhausting process canbe carried out.

In a practical embodiment of the lamp according to the invention twoparallel exhaust tubes located diametrically opposite each other arefixed to the conical collar. The exhausting process can then be carriedout in an efficient manner by introducing a quantity of gas into thelamp vessel via a first exhaust tube and by removing unwanted gases viathe second exhaust tube.

In a special embodiment one of the exhaust tubes is provided with anamalgam to control the mercury vapour pressure during operation of thelamp. In order to prevent the amalgam from reaching the lamp vessel, theopening in the bottom art of the recess associated with that exhausttube is relatively small. Due to the substantially horizontallyextending shape of the bottom art the size of the opening can beadjusted in a simple manner, for example, by punching a softened spot inthe bottom part with a pin having a defined diameter.

The invention will be be described in greater detail with reference tothe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an embodiment of an electrodeless low-pressure mercuryvapour discharge lamp according to the invention and

FIG. 2 shows the sealing member of the lamp vessel of the lamp of FIG. 1in detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The lamp according to FIG. 1 comprises a glass lamp vessel 1 which issealed in a gas-tight manner and is filled with mercury and a rare gas.The lamp vessel has a glass envelope 2 whose inner wall is provided witha luminescent coating (not shown) and whose lower side has a recess witha circular raised edge 2a. The edge of a conically widening collar 3 issecured at the area of the edge 2a in a gas-tight manner by means ofsealing glass ("solder glass"), which collar is located at the end of atubular part 4 which is sealed at its other end (the upper side)5. Thetubular part 4 and the collar 3 form one integral assembly, sometimesreferred to as the stem. This stem is actually the sealing member 6 (seeFIG. 2) for the envelope-shaped part 2 of the lamp vessel. The tubularpart 4 accommodates a rod-shaped core 7 of ferrite. A winding 8 having anumber of copper wire turns is provided around this core 7. The supplywires 9 and 10 of the winding 8 are electrically connected to ahigh-frequency supply unit which is accommodated in the housing 11.During operation of the lamp an electric field is generated in the lampvessel causing a discharge. The housing 11 is surrounded by a syntheticmaterial wall part 12 which is secured to the lamp vessel 1 at one endand is provided with an Edison lamp cap 13 at its other end.

Cam-shaped recesses 14 and 15 by means of which two tubular exhausttubes 16 and 17, respectively, are fixed to the collar wall 3 by meansof a fusing process are provided in the wall of the conical collar 3 intwo positions which are located diametrically opposite each other. Tothis end each cam-shaped recess is provided with a flat bottom partextending at right angles to the longitudinal axis 22 of the tubularpart of the sealing member 6. This is shown in detail in FIG. 2. As isevident from FIG. 2, axis 22 is substantially the longitudinal axis ofthe lamp.

The bottom part of recess 14 is denoted by 14a (see FIG. 2) and that ofrecess 15 is denoted by 15a. The end of an exhaust tube is positionedagainst each bottom part. By means of an orifice in each bottom part thebores of the exhaust tubes are connected to the space within the lampvessel.

In the embodiment shown the lamp vessel is exhausted via exhaust tube16. (In the drawing this exhaust tube is shown with an open end). Arelatively large orifice 18 in the bottom part 14a is present in thecollar. The exhaust tube 17 is shown already sealed and provided with avapour-pressure controlling amalgam 20 (In-Bi-Hg). To prevent thisamalgam (which in solid form has the shape of a rod) from reaching thelamp vessel, the orifice 19 in bottom art 15a is much narrower thanorifice 18 in the bottom part 14a. (The diameter orifice 19 isapproximately 0.5 mm; the internal diameter of the exhaust tube isapproximately 3.2 mm). Furthermore a metal plate-shaped mercury capsule21 which is fixed in the exhaust tube in known manner (see GB-PS No.1,475,458,) is present in the exhaust tube 16. After exhausing, thisexhaust tube 16 is sealed and the mercury is released from capsule 21.Furthermore the lamp vessel contains an In-starting amalgam 23 (seeFIG. 1) on a gauze-shaped holder.

The diameter of orifice 19 is preferably between 0.3 and 0.7 mm. Forsmaller diameters insufficient mercury vapour diffuses to the lampvessel. If the diameter is too large, there is a risk that amalgam 20,which preferably is a pellet with a diameter of about 1 mm in a ductilestate, could pass into the lamp vessel through opening 19 duringoperation of the lamp.

The lamp according to FIG. 1 has a luminous efficiency of approximately1200 lumen at a power supply of 17 W. The luminescent coating consistsof a mixture of green-luminescing terbium-activating cerium magnesiumaluminate and red-luminescing yttrium oxide activated by trivalenteuropium. A transparent conducting layer (not shown) of fluorine-dopedtin oxide was provided between this luminescent coating and the wall ofthe lamp vessel to reduce interference currents in the supply mainsduring operation of the lamp.

What is claimed is:
 1. In an electrodeless low-pressure discharge lampof the type having a discharge vessel defining a lamp axis comprising anouter envelope with an open end and a stem sealed in a gas tight mannerto said outer envelope at said open end, said stem comprising a tubularsection extending along said lamp axis within said outer envelope and aconical collar extending between the end of the tubular section nearsaid open end and said open end, and means for generating a dischargewithin said discharge vessel during lamp operation, said meanscomprising a magnetic core accommodated within said tubular section, theimprovement comprising:said conical collar comprising a recess having aplanar portion transverse to the lamp axis; an exhaust tube parallel tosaid lamp axis having an end joined to said transverse portion; and anorifice in said transverse portion positioned within the inner peripheryof the exhaust tube for allowing communication between the exhaust tubeand the interior space of the discharge vessel.
 2. An electrodelesslow-pressure discharge lamp as claimed in claim 1, wherein said collarcomprises two of said recesses, and two exhaust tubes locateddiametrically opposite each other are fixed to a respective transverseplanar surface on said conical collar.
 3. An electrodeless low-pressuredischarge lamp as claimed in claim 2, wherein one of said exhaust tubesis provided with an amalgam to control the mercury vapour pressureduring operation of the lamp.
 4. An electrodeless low-pressure dischargelamp as claimed in claim 3, wherein the orifice in the planar surface onwhich the end of the exhaust tube provided with the amalgam terminatesis circular and has a diameter of 0.3 to 0.7 mm.
 5. A lamp as claimed inclaim 3, wherein a metal plate-shaped capsule containing mercury issecured in the other of said exhaust tubes.
 6. A method of joining anexhaust tube to a glass surface slanted with respect to the longitudinalaxis of a lamp, comprising:(a) softening a portion of said slantedsurface by heating; (b) forming a planar surface in said softenedportion transverse to the longitudinal lamp axis; (c) fusing the end ofan exhaust tube to said transverse planar surface with the exhaust tubeparallel to the lamp longitudinal axis; and (d) forming an orifice insaid perpendicular planar surface positioned within the base of theexhaust tube.